The present invention relates to an electric power steering apparatus having a ball screw device.
A conventional rack assist type electric power steering (EPS) apparatus includes a hollow shaft, a rack shaft passed through the hollow shaft, and a ball screw device serving as conversion means. When a motor is activated and rotates the hollow shaft, the ball screw device converts rotation of the hollow shaft into reciprocation of the rack shaft. The EPS apparatus thus applies assist force to a steering system.
In the EPS apparatus, the ball screw device has a helical raceway and a plurality of balls each serving as a roller body, which are accommodated in the raceway. The raceway is formed by arranging a threaded groove formed in the outer circumferential surface of the rack shaft and a threaded groove formed in an inner circumferential surface of a ball screw nut, facing each other. For example, in an EPS apparatus described in Japanese Laid-Open Patent Publication No. 2006-256414, a ball screw nut is fixed to an inner circumferential surface of a motor shaft configured as the aforementioned hollow shaft. When the ball screw nut rotates integrally with the motor shaft, the EPS apparatus converts torque produced by the motor shaft into an axial assist force and transmits the force to a rack shaft.
When the ball screw nut is fixed to the inner circumferential surface of the motor shaft, the ball screw nut is clamped by a locknut in an axial direction. However, the fixing structure that fastens the ball screw nut using the locknut may possibly deform the ball screw nut due to the pressing force produced by the locknut. If this causes distortion of the raceway, which is formed by a threaded groove formed in the ball screw nut, smooth rolling of balls may be hampered. Further, noise may be caused by the ball screw nut, and the steering feel may be deteriorated.
To solve this problem, the ball screw nut and the motor shaft may be fastened to each other by a method different from the above-described method. For example, as illustrated in FIG. 11, flanges 73, 74 each extending radially are formed at an end of the ball screw nut 71 and a corresponding end of a motor shaft 72. By fastening the flanges 73, 74 together, the ball screw nut 71 is fixed to the end of the motor shaft 72. This fixes the ball screw nut 71 to the motor shaft 72 in such a manner that the ball screw nut 71 is prohibited from rotating relative to the motor shaft 72, without deforming the ball screw nut 71.
In the rack assist type EPS apparatus configured as described above, the hollow shaft is mounted in a tubular housing by passing the hollow shaft through the housing in the axial direction. The hollow shaft is supported by a bearing arranged between the hollow shaft and the housing. The hollow shaft is thus rotatably received in the housing. Accordingly, if facilitation of the mounting of the hollow shaft is considered, it is desirable that a radially projecting body, such as the aforementioned flanges, be avoided as much as possible. Particularly, in the configuration in which the hollow shaft is formed by the motor shaft, as in the case of the EPS apparatus described in the aforementioned document, the motor shaft, which forms a rotor, is assembled with a stator mounted in the housing. In this case, since the clearance between the housing and the hollow shaft is extremely small, it is substantially impossible to form the flanges integrally with the hollow shaft.
To solve this problem, with reference to FIG. 12, a flange 74 for coupling the ball screw nut 71 may be formed as a flange member 75 independent from a motor shaft 72. In this case, after the motor shaft 72 is passed through the housing, the flange member 75 is coupled to the corresponding end of the motor shaft 72.
Specifically, in the example illustrated in FIG. 12, the flange member 75 has an annular portion 76 functioning as the flange 74 to which the above-described ball screw nut 71 is coupled. The annular portion 76 has an insertion hole 77 through which a rack shaft is inserted. An internal thread 78 is formed on the inner wall surface of the insertion hole 77. An external thread 79 corresponding to the internal thread 78 of the flange member 75 is formed on the outer circumferential surface of the corresponding end of the motor shaft 72. By screw engagement between the internal and external threads 78, 79, the flange member 75 is screwed to the motor shaft 72.
Typically, in an EPS apparatus, the rotating direction of the motor is switched frequently. Accordingly, if a clearance exists in a joint portion between the motor shaft 72 and the flange member 75 in a circumferential direction, the clearance causes chattering in rotation. This produces hitting sound each time the rotation of the motor is switched. However, the above-described fixing structure through screwing prevents the clearance from being created in the joint portion in the circumferential direction. This facilitates the installation of the motor without reducing the quietness.
However, in the structure fixing the flange member through screwing, switching of the rotating direction of the motor, which repeatedly occurs as has been described, produces loosening force that loosens the flange member. This makes it important to prevent such screw loosening of the flange member and, in this regard, the fixing structure has yet to be improved.